If you compare the specs of an S-10 frame to the 3/4-ton frame its only the wheelbase that is a real problem. The later C-10 will give problems with width. Perhaps not insurmountable, but will add to the complications.
And I say that with a C-20 IFS setting next to my 1-ton frame and every intention of mounting it knowing I'll have width problems.
Every change you make compounds the adaptations and fabrication required. If you are not prepared to do those things yourself, you need to be prepared to pay someone else to do them.
Clearly others disagree, but unless your intended usage is defined differently than I've heard, or your confidence at fabrication increases, you have limited choices and I like fixing the stock frame though I see three options that seem consistent with the parameters I've seen you state.
1) Replace with a good stock frame. Lots of work but really just work, nothing tricky and no real special tools required if you are prepared to replace a motor.
2) Fix your frame. Lots of work, but welding and grinding just take time...
3) Go the S-10 route with a kit, and shorten your bed by buying short bed sides, or adjust the length of one of the longer S-10 frames.
The other frame swap choices are all much more complicated mostly because far as I know there are no kits. I suppose another approach would be a stock 1/2-ton frame again shortening the bed, which would not require a kit, but would mean lots of changes from what you have.
You have a number of options presented by me and others. Make a list and review the tools/skills required and the other advantages/disadvantages of each option. It may become real clear that some of the options, even those I suggest, won't work for you, but others fit real well.
Thinking and planning are key to success on a project like this. I always want to encourage people to fully understand what a successful finished project would be before they decide how to proceed.
